Techniques for depositing metals on to surfaces are of great importance for many applications, including the fabrication of electronic devices. This has resulted in the evolution of several enabling techniques, of which electroless deposition is well-established. In this case the substrate is coated first with a catalytic layer. This is done conventionally by immersing the substrate in a solution containing a divalent tin (Sn2+) salt, followed by immersion in a solution containing a palladium salt. The palladium ions are thereby reduced to metallic palladium in a redox reaction which involves the oxidation of Sn2+ to Sn4+. The substrate is subsequently washed to remove residual tin ions, then immersed in a specially formulated solution containing a salt of the metal to be deposited, together with a reducing agent, such as formaldehyde. The palladium catalytic sites on the surface of the substrate facilitate the reduction of the metal salt, causing a metallic film to be deposited on to the substrate.
The production of catalytic coatings prior to electroless plating has been the subject of a great deal of research activity in the past. Two of the objectives of previous inventions were: (1) how to activate the catalyst prior to electroless plating; and (2) how to ensure that the electroless plating bath does not become contaminated with the catalyst. The first of these objectives is discussed in EP0271466, in which the catalyst is associated with a polymeric species; and the relative merits of activating the resultant catalyst-containing coating with ultra-violet light and heat are disclosed. The second objective is addressed in U.S. Pat. No. 5,424,009, and describes also the advantages of associating the catalytic salt with a polymeric species to prevent contamination of the plating bath with the catalyst. In both cases it is necessary to activate the catalyst prior to immersing it in the electroless plating bath.
It was recognised in European patent number EP1436445 that the electroless deposition process could be improved substantially if the catalyst was applied to the substrate as a thin film, in which the catalyst was dispersed within a porous ceramic-type matrix. There are several advantages associated with this approach. For example, the catalyst can be activated simply by heating; while the porous ceramic-type matrix can be selected to achieve strong adhesion to the substrate. The result is that when the electroless deposition process is applied to the coated substrate, the deposited metal builds up from the catalytic nuclei located within the porous matrix, thereby producing a metal film which is strongly bonded to the substrate. In this process organic precursors of both the ceramic-type matrix and the catalytic particles were applied to the substrate as a coating, then heated to convert it to an active catalyst, suitable for subsequent electroless deposition. In EP1436445 an example is described in which this heating step is conducted at 350° C., in order to achieve the objective of a strongly bonded copper film onto a glass substrate. This process works well for heat-resistant substrates, including glasses and ceramics, but it is inappropriate for substrates such as epoxy resins or certain types of plastic films, which can be damaged by exposure to the temperatures required to activate the catalyst and to form the ceramic coating.